Christian Presuppositionalism: buh?

I recently had a random encounter with a proponent of Christian presuppositionalism, over on the Premier Christian Radio forums. Presuppositionalism is a pretty odd position: not content with pointing out the evidence in favour of Christianity, as most Christians do, the presuppositionalist apparently reckons that unless you presuppose the existence of the Christian God, you can't possibly make any sense of the world at all. The original thread the discussion was on got very convoluted (not helped by Ning's limit on thread nesting depth: if you ever want to start a social network, don't use Ning, it's crap). I started another one over here. My initial posting is below, for my reference and also because I'd like to know what you guys think of it. I'm an amateur philosopher, but I know there are some pros reading. Here's what I said:

I'd like to clear the decks a bit rather than arguing in circles. I'm kind of new to presuppostionalism. I used to be a reformed evangelical, but more of an evidentialist (now I'm an atheist, as you might have gathered). Here's what I've managed to glean so far:

Presuppositionalism (as advocated by Sye, at least) seems to be the position that it is necessary to presuppose that the Christian God exists in order to make any sense of the world at all. On this view, the Christian God is the only possible explanation for various stuff which we need for the world to make sense, like logic, mathematics, and our apparent ability to reason from specific cases to general cases ("all the copper we've seen conducts electricity, so all copper conducts electricity"). (This sort of reasoning is usually referred to by philosophers of science as "induction", but note that it's not the same thing as "proof by induction" technique you may have learned in maths lessons). I'll refer to these things as "logic and stuff".

( Now read on... )

To sum up, presup seems a vain attempt to avoid the problem that every theory of knowledge has to start somewhere (or be circular or infinitely recursive, I suppose) by grounding the starting point in God. However, I think I have more confidence that, say, logic "works" than we do in why. I drive my car without knowing what's under the bonnet, and, unless there's a Cartesian daemon deluding my senses, it apparently still gets me from A to B. I might claim that it runs on petrol (gasoline) or on batteries or on some as yet unknown technology, Sye might claim it runs on God. Yet the world is as it is: my car runs on something, and if it isn't God, it's something else.